The present invention generally relates to lasers, and more particularly, to a neodymium laser tuned to emit pulsed light having a wavelength of about 0.9 .mu.m using an optical filter that includes a polarizer and wave plate in the optical resonant cavity of the laser which selectively provides loss for wavelengths in the 1 .mu.m band.
It is relatively easy to optically pump the .sup.4 F.sub.3/2 level of trivalent neodymium doped into various host materials using either flash lamps or more recently, laser diodes. A population inversion between the .sup.4 F.sub.3/2 level and the lower .sup.4 I.sub.9/2, .sup.4 I.sub.11/2, and .sup.4 I .sub.13/2 levels can be readily obtained and laser emission between these levels has been demonstrated. The specific emission wavelengths depend on the host material and are typically near 0.9 .mu.m, 1 .mu.m, and 1.3 .mu.m, respectively. The peak effective cross section is also host dependent and is typically about 10.times.times larger in the 1 .mu.m band. than in the 0.9 .mu.m band. Since these transitions all originate from the same upper level, the relative gain in a particular host at each of these wavelengths is fixed.
A neodymium laser is typically operated in the 0.9 .mu.m band by using wavelength selective elements inside the laser cavity to suppress the stronger 1 .mu.m band emissions. Reflective mirror coatings are designed to provide the required feedback at the desired wavelength and to be highly transmissive over the 1 .mu.m band. This approach is generally adequate when the gain is not large such as the case with continuous wave or long pulse lasers. However, as the inversion increases, more loss is required over the 1 .mu.m band in order to prevent parasitic lasing at that wavelength. This becomes more important with Q-switched operation. The degree of wavelength selective discrimination in the laser cavity effectively sets an upper limit on the energy which can be stored in the laser material.
At present, there do not exist adequate methods to provide the necessary wavelength discrimination for high power Q-switch operation of neodymium lasers in the 0.9 .mu.m band. A number of standard approaches are available for general wavelength selection in lasers. Reflective coatings designed to give the best discrimination possible against unwanted wavelengths are generally used but it is difficult to design coatings to discriminate between closely spaced wavelengths. Other common techniques are also deficient for use in generating a high energy, Q-switched output light at 0.9 .mu.m from a neodymium laser. For example, it is difficult to obtain sufficient wavelength dispersion using refractive elements such as prisms inside the laser cavity. Also, diffraction gratings generally cannot handle high power and tend to introduce significant loss for all wavelengths. Absorption filters which exhibit high transmission at 0.9 .mu.m and sufficient absorption over the entire 1 .mu.m band are not available. Generic birefringent tuning elements are routinely used for wavelength tuning of low-gain lasers. These devices consist of multiple crystal quartz plates having integral multiples of a common thickness. They allow convenient wavelength tuning by plate rotation but are not able to provide the necessary discrimination over the entire 1 .mu.m band. Therefore, a need exists for an intracavity filter which efficiently suppresses 1 .mu.m band laser action and allows efficient operation of Q-switched laser operation in the 0.9 .mu.m band.